1. Field of the Invention
The invention relates in general to a recoating system for use by any solid freeform fabrication technique and, in particular, to a recoating system capable of establishing a uniform layer of a high viscosity build material prior to being solidified by a solid freeform fabrication apparatus. The recoating system is unique in that previously formed layers are not substantially disturbed when applying a new layer of build material to establish a layer of high viscosity build material.
2. Description of the Prior Art
Recently, several new technologies have been developed for the rapid creation of models, prototypes, and parts for limited run manufacturing. These new technologies can generally be described as Solid Freeform Fabrication techniques, herein referred to as xe2x80x9cSFF.xe2x80x9d Some SFF techniques include stereolithography, selective deposition modeling, laminated object manufacturing, selective phase area deposition, multi-phase jet solidification, ballistic particle manufacturing, fused deposition modeling, particle deposition, selective laser sintering, and the like. Generally in SFF techniques, complex parts are produced from a modeling material in an additive fashion as opposed to traditional fabrication techniques, which are generally subtractive in nature. For example, in traditional fabrication techniques material is removed by machining operations or shaped in a die or mold to near net shape and then trimmed. In contrast, additive fabrication techniques incrementally add portions of a build material to specific locations, layer by layer, in order to build a complex part. The additive process varies depending on the technique used, whether by selective deposition of a build material or by selective solidification of a build material.
SFF technologies typically utilize a computer graphic representation of a part and a supply of a build material to fabricate the part in successive layers. SFF technologies have many advantages over the prior conventional manufacturing methods. For instance, SFF technologies dramatically shorten the time to develop prototype parts and can quickly produce limited numbers of parts in rapid manufacturing processes. They also eliminate the need for complex tooling and machining associated with-the prior conventional manufacturing methods, including the need to create molds in casting operations. In addition, SFF technologies are advantageous because customized objects can be produced quickly by processing computer graphic data.
There are a wide variety of build materials that are used in various SEE techniques. These materials are typically applied in the form of a powder, liquid, paste, foam, or gel. Recently, there has developed an interest in utilizing highly viscous paste materials in SEE processes. One of the main purposes of using paste materials is to take advantage of their unique material properties which result in improved properties of the resultant parts formed. These pastes may be obtained by blending a solid charge or filler material in the form of a particulate or powder with a bonding agent. For some pastes the bonding agent is comprised of a photosensitive or heat-cured liquid resin, such as a photopolymerizable resin composed of various combinations of acrylates, epoxies, and vinyl ethers. The powders, typically having a particle size of less than 45 xcexcm, may be a polymer, mineral, metallic, ceramic, or any combination thereof. Some polymer powders that may be used are thermoplastics such as ABS. Nylon, polypropylene, polycarbonate, polyethcrsulfate, and the like. Some metallic powders that may be used are steel, steel alloy, stainless steel, aluminum, aluminum alloy, titanium, titanium alloy, copper, tungsten, tungsten carbide, molybdenum, nickel alloy, lanthanum, hafnium, tantalum, rhenium, rubidium, bismuth, cadmium, indium, tin, zinc, cobalt, manganese, chromium, gold, silver, and the like. Some ceramics that may be used are aluminum nitride, aluminum oxide, calcium carbonate, fluoride, magnesium oxide, silicon carbide, silicon dioxide, silicon nitride, titanium carbide, titanium earbonitride, titanium diboride, titanium dioxide, tungsten carbide, tungsten trioxide, zirconia, and zinc suiphide, and the like. Some rare earth mineral powders thai may be used are cerium oxide, dysprosium oxide, erbium oxide, gadolinium oxide, holmium oxide, lutetium oxide, samarium oxide, terbium oxide, yttrium oxide, and the like. Alternatively, these pastes may also be obtained by blending high viscosity photosensitive or heat-cured liquid resins without a solid charge of filler material, These high viscosity liquids or pastes can be obtained by blending, for example, photopolymerizable resins composed of acrylates, epoxies, and/or vinyl ethers wit any desired toughening agent, such as, for example, polybutadiene, polyethylene, fiberglass, and the like.
The pastes, typically having a viscosity of greater than 10,000 centipoise at ambient conditions, are selectively cured layer by layer by exposure to radiation. Generally, the radiation cures the bonding agent in the paste. The uncured pastes may exhibit a linear stress-strain relationship, a Bingham type linear stress-strain relationship having a threshold yield stress to overcome, a non-linear pseudoplastic stress-strain relationship (shear thinning), or a non-linear dilatant fluid stress-strain relationship (shear thickening). It has been discovered that pastes have significant advantages over other materials used in SFF techniques. For example, the pastes can contain concentrations of a solid charge material, such as a metallic powder, of greater than 50% by volume, which in turn can produce extremely dense green parts. These green parts are well suited for further post processing, such as sintering and infiltration, to produce mechanical properties in the resultant parts that are substantially similar to those achieved by conventional forming techniques such as casting or forging. Thus, it is believed that the utilization of pastes is a significant step forward in achieving rapid manufacturing by solid freeform fabrication techniques.
Recently, there has also developed an interest in utilizing highly viscous liquid materials in SFF processes. For example, in stereolithography a liquid photopolymer resin is used comprising both high and low molecular weight monomers and oligomers. When solidified, the high molecular weight monomers and oligomers provide greater mechanical properties in the resultant objects than the low molecular weight monomers. Thus, it is desirable to maximize the quantity of high molecular weight monomers and oligomers in the resin in order to increase the mechanical properties of the parts formed, and/or include toughening agents to increase the properties. However, when the quantity of high molecular weight monomers and oligomers are increased in a liquid photopolymer resin, the viscosity of resin is also increased. Undesirably, the increase can far exceed the acceptable viscosity range of the resin coating system, since most conventional stereolithography resin coating systems are generally able to work only with low viscosity liquid resins whose behavior is similar to that of a Newtonian liquid. In order to compensate for this, current liquid photopolymer resins used in stereolithography include low molecular weight monomers so as keep the viscosity of the resin within the acceptable viscosity range of the resin coating system. Thus, there is a need to be able to work with high viscosity liquids in order to substantially enhance the mechanical properties of objects formed from liquid photopolymer resins used in stereolithography.
There are number of difficulties that must be overcome when working with high viscosity pastes and liquids in SFF techniques. For example, in order to make a highly viscous material flow, the material must be subjected to a significant amount of shear stress. When attempting to form a uniform layer of a highly viscous material in SFF, the recoater or spreading device invariably induces a significant amount of shear stress on the material. When forming a new layer, the induced shear stress also acts on previously formed layers and can cause the layers of the part to deform, curl, or shift. For highly viscous materials this can undesirably result in de-lamination between layers, missing portions of layers, uneven coating of layers, and the like. In addition, the shear stress induced by the recoater is also problematic in regions before and after the recoater encounters solidified portions of the part being built, such as in regions of xe2x80x9ctrapped volumesxe2x80x9d and other part features. Some previous attempts to solve the problem have focused on controlling the application of shear forces on the material during the coating process. For example, the recoating system in WO 00/51809 utilizes rotating roller members to control the shear forces induced when applying a layer of highly viscous build material. However, highly viscous liquids and pastes are extremely sensitive to the amount of shear stress necessary to properly form a uniform layer, and the proper amount of shear stress to be applied will vary substantially depending on the particular formulation being used. Often, it is necessary to provide a significant number of attachment supports to not only connect the part to the build platform, but also to keep the layers of the part from moving and distorting during the building process. Thus, the use of highly viscous materials in SFF processes is severely limited by current recoating systems.
Another approach to the problem is disclosed in U.S. Pat. No. 5,474,719 to Fan et al. where it is proposed to formulate a viscosity reducible composition. These compositions reduce in viscosity when they are heated, or when a shear stress is applied. Compositions whose viscosity reduces when shear stress is applied are generally known as pseudoplastic fluids or shear thinning fluids which do not encompass the entire rheological range of paste formulations available. Thus, U.S. Pat. No. 5,474,719 to Fan et al. suggests formulating heat liquifable plastic flow compositions and applying them in a heated liquid state, or formulating Bingham type compositions that exhibit shear thinning characteristics and inducing a shear stress to the composition during coating. However, both of these solutions limit the rheological range of materials that can be used. For example, pseudoplastic compositions require special formulation which cannot be achieved for all paste formulations. In addition, because the behavior of paste formulations when subjected to a specific amount of applied shear stress varies, it is extremely difficult to configure a specific shear inducing applicator to work satisfactorily for more than one formulation. Thus, there is a need to provide a better solution to the problem of coating high viscosity materials in SFF techniques, one that does not place undesirable rheological limitations on the material formulations to satisfy coating requirements, or require constant adjustments to the coating applicator.
Also, somewhat pertinent to the present invention are the processes disclosed in U.S. Pat. Nos. 3,264,103 and 3,395,014 to Cohen et. al, developed in the field of tape casting. In these patents, processes are disclosed in which a thermoplastic photohardenable composition is applied, in thin layers as a solution, to a film and then allowed to dry. This coated film is then exposed with UV or visible light imagewise from the film side thereby cross-linking the exposed portions. However, these processes are directed primarily toward the production of printing plates, where the composition is applied on top of a stationary layer of film, and does not address the problem of dispensing over a non-stationary layer of flowable material, such as a paste or thixotropic material, without damaging the non-stationary layer below by shear stresses induced during recoating. A more recent example of tape casting utilizing an evaporative solution is U.S. Pat. No. 6,180,188 to Belleville et al. disclosing optical coatings on stationary surfaces.
Photohardenable compositions present a further problem when attempting to achieve high packing densities in filled materials since the photoinitiated reaction employs ultraviolet (UV) light. Where metals are used as the filler material, such as in the production of a metallic part or a metal tooling part, for example, an aluminum part, the UV light will be attenuated by absorption into the metal filled material or by reflection off of the metal filler. This will preclude the binder from hardening throughout the layer of material. Layers, for example, can be 50 microns in thickness so the depth of cure must be greater than the thickness of the material layer to bond with the underlying material. However, high packing densities of metallic filled binder materials can employ bi- and tri-modal metal particles. These particles will effectively act as blocking agents for the transmission of UV light and thereby prevent the layer to layer boundaries of the part from being completely bonded together.
These and other difficulties of the prior art have been overcome according to the present invention by using a viscosity modified build material to build a three-dimensional object in a layerwise fashion.
The present invention provides its benefits across a broad spectrum of SFF processes by providing the ability to establish uniform layers of a high viscosity build material that solidify via latent polymerization in order to build a three-dimensional object in a layerwise fashion with enhanced mechanical properties.
It is one aspect of the present invention to provide a new method of building a three-dimensional object by SFF that is able to establish uniform layers of a highly viscous build material that overcomes the above-mentioned disadvantages of the prior art.
It is another aspect of the present invention to provide a new method of applying a coating of build material that eliminates the material constraints imposed by current recoating systems and is capable of forming uniform layers of the build material for a greater rheological range of build materials for use in SFF.
It is a feature of the present invention that the build material is transformed by the SFF apparatus in three separate phase states.
It is another feature of the present invention to introduce a viscosity modifier in the build material to significantly reduce the viscosity of the material prior to dispensing the material on a working surface.
It is still another feature of the present invention to significantly increase the viscosity of the material by extracting the viscosity modifier from the build material after dispensing the material on a working surface.
It is still yet another feature of the present invention to extract the viscosity modifier from the build material by providing thermal or heat energy to the material to evaporate the viscosity modifier.
It is an advantage of the present invention that the part being built becomes substantially self-supporting and is not substantially affected by the recoating process due to the modifier induced viscosity reduction of the build material during the recoating process.
It is another advantage of the present invention that the need for supports is substantially eliminated thereby greatly improving the downfacing surfaces of the objects formed and minimizing the need for post-finishing.
It is yet another advantage of the present invention method and apparatus that highly filled dense green parts can be formed from paste-like build materials in SFF for rapid manufacturing.
These and other aspects, features, and advantages are achieved/attained in the method and apparatus of the present invention that employs a build material that is applied by a recoating system in a low viscosity state to a working surface to form a layer of build material having a first viscosity value. The method and apparatus transforms the layer of build material from the low viscosity state to a high viscosity state having a second viscosity value. Preferably the transformation is accomplished by removing a viscosity modifier from the layer of build material by applying thermal heat to evaporate the viscosity modifier. In one embodiment, the first viscosity value of the build material is less than at least one-half of the second viscosity value. In another embodiment, the transformation from the low viscosity state to the high viscosity state is of an amount sufficient that when applying another layer of the build material in the low viscosity state over the layer of build material in the high viscosity state, the transference of shear stress to the layer of build material in the high viscosity state is substantially prevented. In another embodiment, the transformation from the low viscosity state to the high viscosity state is such that the second viscosity value of the build material is raised at least to a point where the build material in the layer is substantially self-supporting.
A build material for use in the present invention comprises a combination of a solid charge or filler material and a binder material that forms a paste having a viscosity of approximately greater than 10,000 centipoise at ambient conditions. The solid charge or particulate matter is preferably a powder material selected from any combination of polymer, metal, ceramic, or mineral particles. The average diameter particle size may vary but is preferably approximately less than about 45 xcexcm. The bonding agent or binder is preferably a phototsensitive liquid resin, formulated from an acrylic, epoxy, and/or vinyl ether photopolymerizable based resin, or combination thereof, or a thermally polymerizable material.
The viscosity modifier that is introduced into the build material is preferably a solvent such as an acetone or alcohol which can be removed after being dispensed by evaporation. The staging area wherein the layers are formed may be heated in order to assist or speed up the evaporation process.
In one embodiment, the viscosity modified build material is extruded vertically to a dispensing platform wherein a smoothing member or doctor blade spreads the material over a working surface which may be a previously formed layer or the platform of the SFF apparatus. In an alternative embodiment, the viscosity modified build material is gravity fed to a recoating applicator having a distribution roller, guide blade, skive, and doctor blade. The recoating applicator having the distribution roller is desirable for use with build materials that exhibit liquid-like properties when in the low viscosity state.
Further embodiments include multiple recoating applicators that dispense different build material formulations. In these embodiments multiple layers of different materials such as highly filled metal layers and highly filled ceramic layers can be formed on a three-dimensional object. In addition, these highly filled three-dimensional objects produced by the method and apparatus of the present invention are well suited for further processing, such as sintering and infiltration.